RFR: JDK-8304959: Public API in javafx.css.Match should not return private API class PseudoClassState [v5]

[John Hendrikx]

> The class `PseudoClassState` is private API, but was exposed erroneously in the CSS API. Instead, `Set<PseudoClass>` should have been used. This PR corrects this.

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Add missing null check

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1070/files
  - new: https://git.openjdk.org/jfx/pull/1070/files/144e04cf..d2651894

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1070&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1070&range=03-04

  Stats: 5 lines in 2 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1070.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1070/head:pull/1070

PR: https://git.openjdk.org/jfx/pull/1070